One of the most important properties of a plasma is its density. While this density is in general a point function, it is often adequate to obtain a measure of the density by measuring the average plasma density over a volume. This information is useful where the plasma is uniform in density over the volume or where the density profile over the volume is known by other means. Reliable measurements also require that the method used to measure density not perturb the density in the course of measurement. This is especially critical in determining the properties of low-density plasmas which are taken to be plasmas less than 10.sup.13 particles per cubic centimeter or 10.sup.19 particles per cubic meter. Plasmas in this density range are conveniently measured by several well-known microwave techniques. These include the fundamental-mode method and the microwave interferometer. In the fundamental-mode method a known cavity shape, a known microwave excitation frequency and a knowledge of the density profile of the plasma in the cavity are combined to determine plasma density from the shift of the resonant frequency of the fundamental mode. This method requires precise determination of the frequency shift. The microwave interferometer measures the average value of density by measuring the change in transmission properties along a line of propagation of a measuring microwave field, but has the disadvantage of requiring high precision of frequency stability and good collimation of the microwave beam to avoid exciting cavity resonances that would interfere with the measurement.
It is an object of the present invention to provide a better means of measuring the density of a plasma.
It is a further object of the present invention to provide a means of measuring plasma density without perturbing the plasma.
It is a further object of the present invention to provide a means of measuring the density of a plasma without regard to the shape of the enclosure in which the plasma is contained.
It is a further object of the present invention to provide a means of measuring the density of a plasma with a frequency source of ordinary frequency stability.
Other objects will become apparent in the course of a detailed description of the invention.